Mediator Design Pattern

Video Lecture

Section	Video Links
Mediator Overview
Mediator Use Case

Overview

Objects communicate through the Mediator rather than directly with each other.

As a system evolves and becomes larger and supports more complex functionality and business rules, the problem of communicating between these components becomes more complicated to understand and manage. It may be beneficial to refactor your system to centralize some or all of its functionality via some kind of mediation process.

The mediator pattern is similar to implementing a Facade pattern between your objects and processes. Except that the structure of the Mediator allows multi-directional communication between the objects or processes that would normally be interacting directly with each other.

While the Facade is a structural pattern, and the Mediator also implies structure in the way that it exists between two or more other objects or processes, it also allows changing the behavior of the interaction to make it more cooperative in some way. E.g., the centralization of application logic, managing the routing behavior, caching, logging, etc.

Terminology

Mediator: The coordinator of communications between the components (colleagues).
Colleagues: One of the many types of concrete components that use the mediator.

Mediator UML Diagram

Mediator Pattern UML Diagram

Source Code

In the example concept, there are two colleague classes that use each other's methods. Instead of the Colleagues calling each other's methods directly, they implement the Mediator interface and call each other via the Mediator. Each colleague is designed for a different purpose, but they utilize some related functionality from each other.

The system, in this case, would work without the Mediator, but adding the Mediator would allow extending functionality to a potential third colleague that provides a different service, such as AI analysis or monitoring, without needing to add specific support or knowledge into the two original colleagues.

In this first example the Mediator is structurally acting as a multi-directional relay between the two colleagues.

./mediator/mediator_concept.py

# pylint: disable=too-few-public-methods
"Mediator Concept Sample Code"

class Mediator():
    "The Mediator Concrete Class"

    def __init__(self):
        self.colleague1 = Colleague1()
        self.colleague2 = Colleague2()

    def colleague1_method(self):
        "Calls the method provided by Colleague1"
        return self.colleague1.method_1()

    def colleague2_method(self):
        "Calls the method provided by Colleague2"
        return self.colleague2.method_2()

class Colleague1():
    "This Colleague provides data for Colleague2"

    @staticmethod
    def method_1():
        "A simple method"
        return "Here is the Colleague1 specific data you asked for"

class Colleague2():
    "This Colleague provides data for Colleague1"

    @staticmethod
    def method_2():
        "A simple method"
        return "Here is the Colleague2 specific data you asked for"

# The Client
MEDIATOR = Mediator()

# Colleague1 wants some data from Colleague2
DATA = MEDIATOR.colleague2_method()
print(f"COLLEAGUE1 <--> {DATA}")

# Colleague2 wants some data from Colleague1
DATA = MEDIATOR.colleague1_method()
print(f"COLLEAGUE2 <--> {DATA}")

Output

python ./mediator/mediator_concept.py
COLLEAGUE1 <--> Here is the Colleague2 specific data you asked for
COLLEAGUE2 <--> Here is the Colleague1 specific data you asked for

Example Use Case

In this example use case, we will implement some behavior into the mediation process.

Before the mediation logic is added, consider that the below example is a series of components all subscribed to a central location being the subject. They all implement the Observer pattern.

Each component is updated independently by external forces, but when it has new information, it notifies the subject which in turn then notifies the other subscribed components.

During the synchronization of all the subscribed components, without the extra mediation, the component that provided the new information will receive back the same message that it just notified the subject of. In order to manage the unnecessary duplicate message, the notifications will be mediated to exclude to component where the original message originated from.

Example UML Diagram

Mediator Pattern UML Diagram

Source Code

./mediator/client.py

"The Mediator Use Case Example"
from component import Component
from mediator import Mediator

MEDIATOR = Mediator()
COMPONENT1 = Component(MEDIATOR, "Component1")
COMPONENT2 = Component(MEDIATOR, "Component2")
COMPONENT3 = Component(MEDIATOR, "Component3")
MEDIATOR.add(COMPONENT1)
MEDIATOR.add(COMPONENT2)
MEDIATOR.add(COMPONENT3)

COMPONENT1.notify("data A")
COMPONENT2.notify("data B")
COMPONENT3.notify("data C")

./mediator/component.py

"Each component stays synchronized through a mediator"
from interface_component import IComponent

class Component(IComponent):
    "Each component stays synchronized through a mediator"

    def __init__(self, mediator, name):
        self._mediator = mediator
        self._name = name

    def notify(self, message):
        print(self._name + ": >>> Out >>> : " + message)
        self._mediator.notify(message, self)

    def receive(self, message):
        print(self._name + ": <<< In <<< : " + message)

./mediator/interface_component.py

"An interface that each component will implement"
from abc import ABCMeta, abstractmethod

class IComponent(metaclass=ABCMeta):
    "An interface that each component will implement"

    @staticmethod
    @abstractmethod
    def notify(message):
        "The required notify method"

    @staticmethod
    @abstractmethod
    def receive(message):
        "The required receive method"

./mediator/mediator.py

"The Subject that all components will stay synchronized with"


class Mediator():
    "A Subject whose notify method is mediated"

    def __init__(self):
        self._components = set()

    def add(self, component):
        "Add components"
        self._components.add(component)

    def notify(self, message, originator):
        "Add components except for the originator component"
        for component in self._components:
            if component != originator:
                component.receive(message)

Output

python ./mediator/client.py
Component1: >>> Out >>> : data A
Component2: <<< In <<< : data A
Component3: <<< In <<< : data A
Component2: >>> Out >>> : data B
Component3: <<< In <<< : data B
Component1: <<< In <<< : data B
Component3: >>> Out >>> : data C
Component2: <<< In <<< : data C
Component1: <<< In <<< : data C

Summary

A mediator replaces a structure with many-to-many interactions between its classes and processes, with a one-to-many centralized structure where the interface supports all the methods of the many-to-many structure, but via the mediator component instead.
The mediator pattern encourages usage of shared objects that can now be centrally managed and synchronized.
The mediator pattern creates an abstraction between two or more components that then makes a system easier to understand and manage.
The mediator pattern is similar to the Facade pattern, except the Mediator is expected to transact data both ways between two or more other classes or processes that would normally interact directly with each other.

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